To provide safety for occupants and decrease gas mileage, the demand for high-strength cold rolled steel sheet for use in automobile parts has increased rapidly in recent years. With the current press-forming technique, most of the inside and outside panels of automobiles are made of cold rolled sheets having a tensile strength of from 35 to 50 kg/mm.sup.2 and it is considered very difficult to apply cold rolled sheets having a tensile strength of 60 kg/mm.sup.2 or more to these parts.
High-strength cold rolled steel sheets given high strength by solid solution or precipitation have been developed for use in inside sheets and outside skins, but their high strength is unavoidably accompanied by increased yield point which not only makes press-forming difficult but also increases the tendency of spring-back to which results in low ability to retain the form obtained by pressing. To solve this problem, a high-strength, dual phase structure cold rolled steel sheet having the ferrite phase and martensite phase has been proposed. An annealed product of this steel sheet does not develop yield point-elongation, has low yield ratio and exhibits good ductility, and hence meets the present need of the car manufacturing industry. A high strength range product having a tensile strength of more than 50 kg/mm.sup.2 is fairly easy to make from this dual phase structure steel sheet, but it is not easy to produce a low strength range dual phase structure cold rolled steel sheet of the type contemplated by this invention, i.e. a steel sheet having a tensile strength of the order of 40 to 50 kg/mm.sup.2 and a low yield ratio.